1. Field of the Invention
The present invention generally relates to the field of medical radiography, and more particularly to a method of making an X-ray scatter reducing air cross grid for use in mammographic procedures, a movable or Bucky air cross grid produced by the method and a method for using the air cross grid in a mammographic procedure.
2. Description of the Prior Art
Scattered X-ray radiation (sometimes referred to as secondary or off-axis radiation) is generally a serious problem in the field of radiography. Scattered X-ray radiation is a particularly serious problem in the field of mammography where high contrast mammogram images are required to detect subtle changes in breast tissue.
Prior to the present invention, scattered X-ray radiation in mammography has been reduced through the use of a conventional linear focused scatter-reducing grid. The grid is interposed between the breast and the film-screen and tends to allow only the primary, information-containing radiation to pass to the film-screen while absorbing secondary or scattered radiation which contains no useful information about the breast being irradiated to produce an X-ray image.
Conventional focused grids used in mammography generally comprise a plurality of X-ray opaque lead foil slats spaced apart and held in place by aluminum or fiber interspace filler. In focused grids, each of the lead foil slats, sometimes referred to as lamellae, are inclined relative to the plane of the film so as to be aimed edgewise towards the focal spot of the X-rays emanating from the mammographic X-ray source. Usually, during a mammographic exposure, the standard practice is to move the focused grid in a lateral direction, perpendicular to the lamellae, so as to prevent the formation of a shadow pattern of grid lines on the X-ray image, which would appear if the grid were allowed to remain stationary. Such moving grids are known as Bucky grids.
One problem with conventional mammographic grids of the type described above is that the aluminum or fiber interspace filler material absorbs some of the primary, relatively low energy, information-containing X-ray radiation. Because some of the primary radiation is absorbed by the interspace material, the patient must be exposed (theoretically) to a higher dose of radiation than would be necessary if no grid were in place in order to compensate for the absorption losses imposed by the grid. It is an obvious goal in all radiography applications to expose the patient to the smallest amount of radiation needed to obtain an image having the highest image quality in terms of film blackening and contrast.
Another problem with such conventional focused mammographic grids of the parallel lamellae type described above is that they do not block scattered radiation components moving in a direction substantially parallel to the plane of the lamellae. The resulting images using these grids have less than optimal darkness and contrast.
These and other disadvantages, such as structural weaknesses, with the conventional focused scatter reducing grids of the type described above are set forth in U.S. Pat. Nos. 1,476,048; 2,133,385; and 2,605,427. Several prior art patents, such as U.S. Pat. Nos. 4,288,697; 4,465,540; 4,951,305 have suggested focused crossed-pattern type grid structures to overcome some of the above noted disadvantages with the conventional parallel-lamellae scatter reducing grids described above. For example, U.S. Pat. No. 2,605,427 suggests forming a cross-patterned grid comprising a plate of a substance permeable to X-rays formed, in two preferably perpendicular directions with narrow closely-spaced grooves filled with a substance impermeable to X-rays. The basic problem with the cross-patterned grid described in this U.S. Pat. No. 2,605,427 is that the X-ray permeable materials, similar to the fiber and aluminum interspace materials in conventional mammographic grids currently used, are not totally X-ray permeable, that is, the material still absorbs some of the primary X-rays, resulting in a lower transmission rate of primary X-rays to the X-ray film and leading to higher X-ray dosages to the patient.
U.S. Pat. No. 4,465,540 describes a collimator for a Gamma camera system. The collimator described therein is constructed by assembling a plurality of X-ray collimating layers comprising radiation-absorbent materials having openings etched therein. The collimating layers are assembled in a spaced apart relationship with a plurality of radiation transmissive spacer layers between each collimating layer. The assembly is held together at the periphery by bolts. Several problems arise when attempting to apply this particular construction to make a focused, moving mammographic scatter reducing grid. First, like other grids previously mentioned, there are several layers of X-ray transmissive spacer layers within each focused passage or opening. Although characterized as X-ray transmissive, these layers within the X-ray transmissive openings still absorb more radiation than if the openings were straight through and only filled with air. These layers also present the potential to cause some scatter of X-rays leaving the collimator. Another problem with this construction is that the X-ray absorbing layers in the middle of the collimator are not physically bound together. Although not apparently a problem in Gamma camera systems, this can present a problem in mammography because the common practice is to move the grid during an exposure to blur grid lines, and there is a risk that the unbonded layers could physically separate from one another and distort the resulting image.
Two additional patents, U.S. Pat. Nos. 5,231,654 and 5,231,655, describe computerized tomography (CT) X-ray collimators formed of one or more layers of Corning Fotoform.TM. photosensitive glass, with focused passages etched away to form cross grids with thick glass partitions between the air passages. The effective primary radiation-transmission area of these cross grids is drastically reduced by the radiation absorbing cross-sectional area of the thick glass partitions. Only about 20% to 25% of the grid transmits primary radiation and the X-ray dose absorbed by the patient is correspondingly increased. Therefore, such thick-partition glass grids are totally unsuitable for low energy mammography X-rays requiring maximum image contrast with minimum radiation dosage.